Some currently-implemented wireless communication systems provide for multiple input/multiple output (MIMO) communication between a transmitter with M transmit antennas and a receiver with N receive antennas to improve the capacity and reliability of wireless communication channels. An M×N MIMO system such as this generally provides a linear increase in capacity with K, where K is the rank of the system and is defined as the minimum of the number of transmit antennas and the number of receive antennas (i.e., K=min(M,N)).
However, although a system may be able to support 4×4 MIMO, rank-4 transmissions that include four MIMO layers are not always desirable. The MIMO channel experienced by the user equipment (UE) generally limits the maximum rank that can be used for transmission. In general, for weak users in the system, a lower rank transmission may be preferred over a higher rank transmission from throughput perspective. It is also possible that only a small fraction of UEs in the system may be equipped with four receive antennas.
Another aspect of a MIMO system is the amount of feedback overhead required. In an OFDM MIMO system, for example, multiple channel quality indicators (CQIs) may be required even for single-layer transmission to exploit frequency-selective multi-user scheduling gains. The introduction of multiple MIMO layers may then further increase the feedback overhead. Therefore, there is a need in the art for an improved method for managing channels in a wireless communication system.